Formal and real voting power

The so-called power indices are considered valuable tools for the analysis of various kinds of collective choice. Their applicability for the understanding of legislative decision-making has been demonstrated. If a distinction between formal voting power and real voting power is introduced, then the power index approach to the analysis of ongoing processes of social choice becomes more relevant. The concept of real voting power implies an extension of the power index approach to take into account how actors behave in order to restrict the coalition possibilities. Thus, in a legislative setting it is more suitable for the measurement of actual power, whereas the formal power indices measure constitutional power. The differences between formal and real voting power may be quite substantial as is shown in an application of these concepts to data concerning the Swedish Riksdag during the seventies.